1. Field of the Invention
The present invention relates to an information signal reproduction apparatus and, more particularly, to tracking control upon reproduction.
2. Related Background Art
As a conventional apparatus of this type, a digital VTR is known. In this apparatus, an image signal is converted into a digital signal, the digital signal is encoded to compress its information size, and the encoded digital signal is recorded on a magnetic tape while forming a large number of tracks. Upon reproduction, these tracks are traced by a rotary head to reproduce the recorded image signal, and the reproduced image signal is decoded to expand its information size, thus obtaining an original image. For a digital VTR of this type, in recent years, home-use digital VTR standards called DV format has been proposed.
Of this DV format, in an SD mode, an image signal is converted into a digital signal, its information size is compressed to around ⅙ by a technique such as the DCT, variable-length coding, or the like, and an image signal for one frame is recorded on 10 helical tracks in case of NTSC.
By contrast, in order to record an image signal on a magnetic tape for a longer period of time, a recording mode called an SDL mode has also been proposed. In this mode, a higher compression ratio than the SD mode is set to compress the information size of an image signal to ½, and an image signal for one frame is recorded on five tracks.
As the arrangement of a head for recording/reproducing an image signal in both the SD and SDL modes, the arrangement shown in FIG. 1 may be used.
More specifically, referring to FIG. 1, a magnetic tape T serves as a recording medium, and a rotary drum 101 rotates in contact with the magnetic tape. In FIG. 1, the tape T is wound around the rotary drum 101 over a 180° range or larger. A rotary head (HA1) 102 is attached onto the drum 101. A rotary head (HB1) 103 is attached onto the drum 101 to have a 180° phase difference from the head (HA1) 102. A head (HB2) 104 for the SDL mode is attached in the neighborhood of the head (HA1) 102.
The heads 103 and 104 have the same azimuth angle, and the head 102 has an azimuth angle different from those of the heads 103 and 104.
FIG. 2 shows tracks formed on the magnetic tape T by the heads shown in FIG. 1 in the respective modes.
In the SD mode, the tape T travels at a predetermined speed S in the direction of an arrow A, and a recording current is alternately supplied to the head (HA1) 102 and head (HB1) 103 that trace in the direction of an arrow H, thereby forming tracks at a rate of two tracks per rotation of the rotary drum 101.
Referring to FIG. 2, HA1, HB1, and HB2 indicate the heads that formed tracks, and F0, F1, and F2 the states of tracking signals superposed on the recorded signals on the individual tracks.
In the DV format (SD and SDL modes), an image signal undergoes 24/25 modulation upon recording to superpose a pilot signal used in tracking control upon reproduction on the recorded signal on each track. FIG. 3 shows the frequency components of recorded signals on tracks F0, F1, and F2, and tracks are formed in the order of F0→F1→F0→F2→F0. As shown in FIG. 3, on track F1, a frequency component f1 has a peak, and a frequency component f2 and DC component are suppressed. On track F2, a frequency component f2 has a peak, and a frequency component f1 and DC component are suppressed. On track F0, frequency components f1 and f2, and the DC component are suppressed.
In the SD mode, an image is recorded by forming 10 tracks per frame on the tape T.
On the other hand, in the SDL mode, although the rotational speed of the drum 101 is the same as that in the SD mode, the convey speed of the tape T is set at S/2, i.e., half that in the SD mode, and a recording current is alternately supplied to the head (HA1) 102 and head (HB2) 104, thus forming tracks at a rate of one track per rotation of the rotary drum 101. At this time, since the track pitch is nearly the same as that in the SD mode, and the convey speed of the tape is half that in the SD mode, the track angle becomes slightly different from that in the SD mode.
In the SDL mode, an image signal is recorded by forming five tracks per frame. For this reason, the SDL mode can realize a recording time twice that in the SD mode.
When an image signal is reproduced from the tape T shown in FIG. 2, an image signal recorded in the SD mode is reproduced by the head (HA1) 102 and head (HB1) 103, and an image signal recorded in the SDL mode is reproduced by the head (HA1) 102 and head (HB2) 104. In this case, an image signal is reproduced while controlling tracking between the heads and tracks on the basis of pilot signals superposed upon recording.
As described above, upon reproducing the contents of a tape on which an image signal is recorded in only the SD mode, the head (HA1) 102 and head (HB1) 103 shown in FIG. 1 need only be used; upon reproducing the contents of a tape on which an image signal is recorded in only the SDL mode, the head (HA1) 102 and head (HB2) 104 shown in FIG. 2 need only be used.
However, upon reproducing the contents of a tape on which image signals are recorded in both the SD and SDL modes, as shown in FIG. 2, the three heads shown in FIG. 1 are required, and the number of heads required increases compared to reproduction of a tape on which an image signal is recorded in only the SD or SDL mode.
For this reason, such arrangement leads to an increase in cost due to requirements of extra head, rotary transformer, head amplifier, and the like.